The invention is directed to test equipment for measuring physical characteristics of a specimen. More particularly, the invention relates to compression testing apparatus for measuring specimen deflection and identifying specimen failure when a specimen is subjected to a load. In practice, the pressure applied to the specimen is ordinarily measured while the specimen is being stressed to failure. The accuracy of such measurements depends upon proper alignment of the specimen and the test equipment to assure that the pressure applied to the specimen is uniformly distributed over the area subjected to the compression. To obtain uniform pressure distribution it is necessary to position the specimen in the test equipment in such a manner that the bearing surfaces or platens which engage the specimen therebetween are parallel with the respective surfaces of the specimen with which such platens come into contact. If adjacent surfaces are not properly matched, the portion of the specimen at the point of initial contact by the platen will be subjected to the compressive forces of the test equipment when loading commences, and overloading of the specimen at that point will occur and cause specimen failure at a stress level lower than would have been obtained if the specimen was uniformly loaded. This invention provides improved means to prevent such non-axial loading.
Prior attempts to assure the uniform distribution of pressure applied to a specimen have included the use of what might be referred to as ball-joint engagement of at least one of the load bearing platens of the test equipment. In such equipment at least one of the platens comprises a hemispherical member seated in a convex cavity in a support. In operation, as opposing platens engage opposite ends of the specimen, the hemispherical member moves within the convex cavity in a manner similar to the movement effected between the parts which form a ball-joint structure. It is believed that alignment between the platens and specimen is obtained as a result of the movement or pivoting of the hemispherical member (ball) within the convex cavity (socket) until such time as the load bearing surface associated with the hemispherical member uniformly engages the end of the specimen. One disadvantage of such a mechanism occurs in those instances where equipment is used which requires the manipulation of the specimen by the individual performing the test to obtain proper alignment. In such instances, obtaining alignment is a sensing procedure involving movement of the specimen by hand as the specimen contacting ball-type platen pivots about the socket-type support. The use of such an apparatus may or may not result in perfect seating. Further alignment is not possible after loading commences since to further align the specimen would require lateral translation of one of the platens. Prior art testing equipment does not include this latter feature.
Other prior art test equipment utilizes complex mechanisms. For example, in some equipment alignment between the test specimen and load bearing platens is obtained by subjecting flexible diaphragms to fluid under pressure. Such diaphragms act upon pressure plates which transmit pressure to the specimen to conform to the ends thereof.
Accordingly, it is an object of this invention to provide a testing apparatus which compresses a specimen so that the pressure is uniformly distributed over the area subjected to the compression.
Another object of this invention is to provide a testing apparatus having a specimen contacting platen which may be translated laterally.
Still another object of this invention is to provide a testing apparatus having a specimen contacting platen which may be rotated 360.degree..
A further object of this invention is to provide a testing apparatus which includes a specimen contacting platen having a ball-joint engagement and which also may be subjected to lateral translation and rotated 360.degree..
Still a further object of this invention is to provide a testing apparatus which may be manipulated by the individual performing the test, after loading commences, to automatically align the specimen.
Yet another object of this invention is to provide a testing apparatus which does not comprise a complex mechanism.